The present invention relates to a laser beam type high speed thermal printer (hereinafter referred to as "laser thermal printer") wherein laser beams are radiated to a heat-sensitive recording web, such as heat-sensitive recording paper, to cause thermal development of color.
In the thermal printer designed to cause development of color by microspot-heating heat-sensitive recording paper according to a print pattern, the common heating system is, at present, of the so-called contact type. In this type, very fine heating bodies are arranged along the width of the heat-sensitive recording paper and selectively turned on and off to develop desired letters or figures on the paper being fed. Therefore, this type is liable to malfunction or deteriorate owing to the heat-sensitive recording layer of the heat-sensitive recording paper adhering to or accumulating on the heating bodies (printed head) which contact the heat-sensitive recording layer.
The laser thermal printer of the present invention is intended to solve such problems about the contact type thermal printer. More particularly, according to the laser system, without the use of a printing head or the like physically contacting the heat-sensitive recording layer, laser beams are radiated as micro-spots or dots according to a desired print pattern, their thermal energy being used to print letters (or figures), so that there is no problem of adhesion or accumulation described above.
However, in order to construct a practical laser thermal printer using heat-sensitive recording paper, it is necessary to meet the requirements including satisfactory correspondence between the laser wavelength and the absorption wavelength of the paper, and less decrease in the width of the peak wavelength or less attenuation of the beam caused by the so-called modulation mechanism for turning on and off the beam radiation according to a print pattern. At present, there is no laser thermal printer that satisfies these requirements; only methods proposed as possibilities include one for incorporating a special material in heat-sensitive recording paper which absorbs laser beam so as to make up for the deviation between the laser wavelength and the absorption wavelength of the heat-sensitive recording paper.
In addition, of the presently commercially available printers, those called the laser type are of the electrostatic print type using wood free paper and designed to modulate the output of the He-Ne laser to form an electrostatic latent image on a photo-sensitive drum and to thereby transfer the toner; thus, they have nothing to do with the laser thermal print type of the present invention.
The inventors have made various experiments and studies with attempts to solve the problems encountered in constructing the laser thermal printer described above and have found that if a CO.sub.2 gas laser in which the beam wavelength is within the range of 9.2 to 10.6 .mu.m is used as the laser source, a heat-sensitive color developing layer having a high absorption power can be formed on the heat-sensitive recording paper, making it possible to print letters through direct laser radiation, and sufficiently high speed printing is possible when the laser output power is 10 W or more. On the other hand, it has been found that the He-Ne laser and YAG laser are not for practical use from the standpoint of their wavelength and the output powers obtained.